Sen. John McCain, the Arizona Republican who's a longtime advocate of arming the Syrian rebels, today tweeted and displayed on the Senate floor graphic photos of dead children and other casualties of the nearly 3-year-old civil war in Syria.

McCain said on Twitter and on the Hill that the images "haunt" him but that "what haunts me more is thought that we'll continue to do nothing."

McCain has long criticized the Obama administration's reluctance to become more militarily involved in stopping the bloodshed in Syria, where a popular uprising against President Bashar Assad has turned into a ferocious civil war with Islamist-led rebels facing the regime's military and militias.

The photos, previously reported on by CNN and Britain's Guardian newspaper, purportedly come from a Syrian military policeman's cache of tens of thousands of photographs that were taken between March 2011 and August 2013.

The images show emaciated prisoners, rows of dead toddlers and other evidence of what McCain calls "mass atrocities." At least one reporter has questioned the credibility of the report that accompanied the photos' release, calling it a "well-timed propaganda exercise funded by Qatar."

“When the images and horrors of this conflict occasionally show up on our television screens, the impulse of many Americans is to change the channel,” McCain said on the Senate floor. “But we must not look away. We must not avert our eyes from the suffering of the Syrian people – for, if we do, we ignore, we sacrifice that which is most precious in ourselves: our ability to empathize with the suffering of others, to share it, to acknowledge through our own sense of revulsion that what is happening in Syria today is a stain on our the collective conscience of moral peoples everywhere.

“These images of the human disaster in Syria haunt me. And they should haunt all of my colleagues and all Americans,” McCain later added. “But what haunts me even more than the horror unfolding before our eyes in Syria is the thought that we will continue to do nothing meaningful about it, and how that deadens our national conscience, and how it calls into question the moral sources of our great power and the foundations our global leadership.”

At the State Department, spokeswoman Marie Harf said she hadn't seen McCain's tweets, but supports any effort to show the public the atrocities coming out of Syria. She bristled, however, at McCain's suggestion that the U.S. policy is to do "nothing."

Harf told reporters that the U.S. is "heavily engaged" in trying to find a diplomatic solution for Syria; she said that getting "entrenched" in another war in the Middle East wouldn't serve U.S. interests or values. She cited the chemical weapons deal and the Geneva2 diplomacy track as two ways the U.S. has taken "meaningful" steps toward ending the crisis.

However, Harf added, there aren't a lot of good options for the United States in dealing with a "regime that's willing and able to brutalize its own people."